"Our results suggest that infants, just like adults, are able to tell the difference between those who act positively vs. negatively toward others, and that they tend to approach those who act positively and to avoid those who act negatively," Hamlin tells WebMD via email.

Hamlin's team studied healthy, full-term babies aged 6 and 10 months.

The babies sat in their parents laps and watched a skit in which a red circle with big button eyes tried to climb a steep hill.

Along came a triangle with big beady eyes. Sometimes the triangle helped the circle climb the hill. Other times, the triangle pushed the circle down the hill.

Afterward, the babies had a chance to reach for the helpful triangle or the mean triangle.

Based on which object they reached for, the babies "robustly preferred" the helpful triangles over the mean triangles, write the researchers.

"We were impressed - but not necessarily surprised," says Hamlin.

"We feel that this is likely because being able to distinguish those who may help you from those who may harm you is so important for successful existence in a social world."